A German research ship laden with 20 tonnes of iron sulphate has whipped up a storm of protest as it sails towards the Antarctic, where it intends to dump its cargo into the ocean.

Scientists on thePolarstern, which set sail from Cape Town in South Africa on 7 January, plan an ocean fertilization experiment that some argue will violate international law.

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But the scientists say that it will yield the very data necessary to assess the impact of the controversial geo-engineering technique, which aims to trap carbon dioxide from the atmosphere by encouraging the growth of algae.

The team, comprising about 50 scientists from Germany, India, Italy, Spain, Chile, France and Britain, is heading for a small patch of the Scotia Sea between Argentina and the Antarctic Peninsula. The researchers hope that the iron will induce an algal bloom in this usually nutrient-poor region, and plan to observe the growth and decay of the organisms in unprecedented detail during the following eight weeks.

The experiment is called LOHAFEX, taking its name from loha, the Hindi word for iron. It will be the sixth ocean fertilization study conducted in the Southern Ocean since 1993. Previous experiments, such as the European Iron Fertilization Experiment (EIFEX) carried out in 2004, indicated that iron fertilization could help to send more carbon-based materials down to the deep ocean.